1. Field of the Invention
The present invention relates to a water-cooled four cycle engine, and particularly to a water-cooled four cycle engine suitable for a hybrid type power unit including an electric motor in addition to a gasoline engine. The present invention also relates to a thermostat mounting structure for a water cooled engine, and particularly to a thermostat mounting structure suitable for a motorcycle on which a small-sized water-cooled engine is mounted.
2. Description of Background Art
Vehicles having a gasoline engine as a drive source are mainly available at present; however, vehicles having an electric motor as a drive source are also required at locations where exhaust gas must be avoided. An electric motor-driven vehicle, however, has increased body weight and shortened running distance compared to a gasoline engine driven vehicle. Therefore, it has been increasingly necessary to develop a hybrid type vehicle including an electric motor in addition to an engine.
For example, Japanese Patent Laid-open No. Hei 8-175477 discloses a hybrid type motorcycle entitled "Device for Switching Engine Power to/from Motor Power in Motorcycle or the Like".
In the above-mentioned document, an engine 10 is a simple air-cooled two cycle engine.
In a hybrid type power unit, however, it is desired to mount a water-cooled four cycle engine in consideration of fuel consumption, emission and the like. A water-cooled four cycle engine requires a water pump for water cooling, and a cam shaft drive mechanism for driving a valve mechanism. A water-cooled engine also requires a cooling water line including a radiator, thermostat and the like.
FIG. 14 is a view showing one example of a related art cooling water line for a motorcycle. Referring to FIG. 14, after having been used for cooling a cylinder block 201 and a cylinder head 202, hot water flows to a radiator 204 through a thermostat 203 as shown by a black arrow, being forcibly cooled at the radiator 204. The water thus cooled flows to an engine 200 as shown by a white arrow.
In FIG. 14, 207 is a water pump; 208 is a siphon tube of the radiator 204; 209 is a radiator cap; 210 is a reserve tank; and 211 is an overflow tube of the reserve tank 210.
Since the temperature of cooling water is low immediately after the engine 200 starts to be operated, the thermostat 203 is closed and the cooling water is circulated to the engine 200 not by way of the radiator 204 to promote temperature rising of the engine 200.
In the above example, since the thermostat 203 is required to be contained in a case 205 and covered with a lid 206, the number of parts associated with the thermostat 203 is increased. In a motorcycle having a limitation in its part mounting space, it is desired to reduce the number of parts.
Furthermore, a water pump, a cam shaft and the like are driven by auxiliary power supplied from a crank shaft, and therefore, the weight of the engine becomes large. In particular, in a hybrid type power unit including an electric motor in addition to an engine, the size and weight of the unit increase.